The use of plastic materials, such as fiberglass-reinforced thermosetting polyester resins for exterior auto and truck parts, has steadily increased. These cured resin materials have been successful because they are strong, heat resistant and enable molders to form one plastic part instead of several metal parts. By their very nature, however, thermosetting composites are difficult to mold without seriously compromising the surface cosmetics of the finished part. As consumers demand better quality, material suppliers and molders have worked to continuously improve the surface smoothness and dimensional stability of composite parts.
A number of thermoplastic polymer additives are used to improve surface quality of molded thermosetting plastic parts. These materials are called low profile additives because they decrease surface roughness and improve surface smoothness or "profile" by reducing the shrinkage of the thermosetting resin as a part is cured during molding. When significant cure shrinkage occurs, a cross section of the surface appears rough under high magnification showing large peaks and valleys. When cure shrinkage is reduced, the surface is smoother, and its cross section appears smoother, having lower peaks and shallower valleys. The objective of an effective low profile additive is to reduce the difference between the highest peaks and shallowest valleys to a minimum. Even the best low profile additives however, do not achieve a mirror-like surface.
Examples of thermoplastic polymers used as low profile additives to control reaction shrinkage and improve the surface smoothness when curing unsaturated polyester resins and vinyl ester resins include polystyrene, polyesters, polymethyl methacrylate, polyvinyl acetate, polyurethanes, polyglycols, and their various copolymers.
U.S. Pat. No. 3,959;209 Koppers, lists polymeric thermoplastics which reduce the shrinkage of thermosetting resin formulations as they are cured and thus effectively produce articles having smooth, high quality cosmetic surfaces. Listed in U.S. Pat. No. 3,959,209 are homopolymers of ethylene, styrene, vinyl toluene, alkyl methacrylates, alkyl acrylates, various copolymers of vinyl chloride and vinyl acetate, styrene and acrylonitrile, methyl methacrylate and alkyl esters of acrylic acid, methyl methacrylate and styrene, methyl methacrylate, and acrylamide.
Thermoplastic low profile additives can sometimes improve other properties of thermoset molding compounds. U.S. Pat. No. 4,421,894 Olin describes a polyurethane oligomer modifier for incorporation into thermosetting polyester resin formulations which imparts improved impact properties to the often brittle thermoset resins in addition to reducing shrinkage during curing.
In the Kunststoffe 1983 publication, Krolikowski ranked various thermoplastic additives according to increasing shrinkage values of the corresponding modified unsaturated polyester resin systems: high density polyethylene, vinyl acetate copolymer with diallyl phthalate, polycaprolactone, polystyrene, polymethylmethacrylate, polyhydroxyester, and styrene copolymer with diallyl phthalate.
U.S. Pat. No. 5, 116,917 Ashland Oil describes a saturated polyester low profile additive for vinyl ester resin systems made from dibasic acid and an ethylene oxide/propylene oxide polyether polyol having an ethylene oxide/propylene oxide molar ratio ranging from 0.1 to 0.9.
Among the more versatile low profile additives are thermoplastic polyesters. This versatility is due to the wide variety of polyester copolymers that can easily be prepared, thus polyester low profile additives can be custom designed for a particular function in the unsaturated base resin of interest. Most polyester low profile additives are made from aliphatic dibasic acids or anhydrides such as glutaric, adipic, sebacic or azeleic, and glycols based on ethylene oxide or propylene oxide. These low profile additives are usually easy to prepare and formulate, have good compatibility in styrene and base resins, and show excellent shrinkage control which produces a high quality cosmetic surface.
While less common, when mechanical properties are critical, low profile additives are also prepared from aromatic dibasic acids or arthydrides such as phthalic, isophthalic and terephthalic, and glycols or lactones. Such polyesters are often more difficult to prepare, less soluble in styrene, less compatible in other polyesters, and typically show more cure shrinkage and reduced surface smoothness. However, they are usually tougher, with higher glass transition temperatures and melting points, and are more resistant to transesterification. This toughness yields increased mechanical properties for composite parts, especially at elevated temperatures. As molding formulations are often premixed and stored prior to use, the possibility of transesterification between the low profile additive and base resin always exists. This transesterification results in a drop in the effective concentration of low profile additive, a loss in shrinkage control, and reduced surface smoothness. It would be most desirable to identify and prepare low profile additives from aromatic diacids which would produce composite molding formulations with superior resistance to transesterification, increased mechanical properties, and excellent surface smoothness.
To this end, the low profile additives of this invention are prepared primarily from isophthalic and terephthalic acid and a variety of symmetrical and asymmetrical dihydric alcohols or glycols. These evaluations have lead to the following conclusions:
Resistance to transesterification is primarily a function of the aromatic diacids content and is approximately proportional to the molar ratio of aromatic to aliphatic diacids. Resistance is best when there is a major portion of aromatic diacids. PA1 For polymers with high aromatic diacid content:
compatibility in styrene and unsaturated base resins and effectiveness at reducing cure shrinkage and improving surface smoothness is mainly a function of the variety and structure of glycols used and the number average molecular weight (Mn) of the polymer, PA2 mixtures of glycols containing a substantial amount of asymmetrical materials, such as propylene, dipropylene, polypropylene, and 2-methyl 1,3-propanediol show increased effectiveness when used to prepare low profile additives, PA2 low profile additives prepared using glycol mixtures with substantial quantities of 2-methyl 1,3-propanediol are effective in a wider variety of base resins than other asymmetrical glycols, PA2 for equal low profile additive concentrations, the mechanical properties of the composite increase as the mechanical properties of the low profile additive increase.